GSM - Global System for Mobile Communications

Transcription

1 GSM - Global System for Mobile Communications VLR BTS BSC GMSC PSTN MS HLR 1) Overview of GSM architecture 2) GSM channel structure 05-1 GSM - Global System for Mobile Communications VLR BTS BSC GMSC PSTN MS HLR MS: Mobile Station BTS: Base Tranceiver Station BSC: Base Station Controller GMSC: Gateway Mobile Switching Centre HLR: Home Location Register VLR: Visitor Location Register 05-2

2 GSM Subsystems OSS: Operation Subsystem Control Flow User Data Flow OSS MS BSS NSS MS: Mobile Station BSS: Base Station Subsystem NSS: Network and Switching Subsystem 05-3 GSM: External environment of the BSS Control Flow User Data Flow OSS MS ME+SIM BSS BTS+BSC NSS GMSC+MSC +HLR+VLR ME: Mobile Equipment SIM: Subscriber Identity Module 05-4

3 GSM: Functional architecture and principal interfaces Air Interface A-bis Interface A interface BSS MS BTS BSC NSS MSC/ GMSC HLR MAP MAP VLR Operation, Administration & Maintenance (OAM) 05-5 Mobile Station (MS) The MS is the mobile handset, it contains the ME and the SIM Mobile equipment (ME) mobile handset hardware, including RF, GSM modulation etc Identified by a unique International Mobile Equipment Identity (IMEI) (different from the phone number) Subscriber Identity Module (SIM) contains subscriber-related information Identified by a unique International Mobile Subscriber Identity (IMSI) (different from the phone number) 05-6

4 Base Station Subsystem (BSS) The BSS consists of BTSs and BSCs Base Tranceiver Station (BTS) responsible for communication with the MS responsible for radio transmission and reception includes antennas, modems, signal processing Base Station Controller (BSC) responsible for radio interface management of BTS and MS, i.e. channel management and handovers responsible for communication with the NSS a single BSC typically manages BTSs 05-7 Network and Switching Subsystem (NSS) NSS contains the switching functions of GSM, as well as databases for mobility management NSS contains Mobile Switching Centre (MSC) Gateway MSC (GMSC) Home Location Register (HLR) - co-located with GMSC Visitor Location Register (VLR) - co-located with MSC/GMSC Signalling between MSC, GMSC, HLR, VLR via SS7 signalling network, using specifically the mobile application part (MAP) of Signalling System No 7 (SS7) 05-8

5 NSS cont d... Mobile Switching Centre (MSC) coordinates setup of calls to and from GSM users controls several BSCs Gateway MSC (GMSC) gateway to external network incoming call is routed to GMSC, which then determines MS location GMSC function is often in the same machine as the MSC 05-9 NSS cont d... Home Location Register (HLR) contains information about subscribers, e.g. subscriber profiles, also information on their current location IMSI, user phone number, address of current VLR etc Visitor Location Register (VLR) temporarily stores subscription data for subscribers currently in the (G)MSC area contains more precise location data than does the HLR linked to one or more MSCs 05-10

6 Operation SubSystem (OSS) Network operation and maintenance Subscriber data management Call charging Mobile equipment management via Equipment Identity Register (EIR) EIR HLR VLR BTS BSC GMSC PSTN BTS BTS BTS BSC BSC MSC VLR 05-12

7 GSM Functional Planes Operation, Admin & Mtce operator user Communication Management Mobility Management Radio Resource Management Transmission Management Call Control Mobility and security, HLR and VLR Ensures stable radio connections, includes handover process Physical Layer, coding, modulation, channel multiplexing etc GSM Channel Structure Channel Requirements Traffic Channels Associated Signalling Channels Call-related signalling Common Signalling Channels Cell information channel(s) (downlink) Paging channel (downlink) Access channel(uplink) These channels all need to be efficiently multiplexed into the GSM frame structure 05-14

8 Traffic Channels The GSM channel structure includes three types of physical channel, called traffic channels (TCH): TCH/F TCH/H TCH/8 Full rate traffic channel (13 kbps speech channel) Half rate traffic channel (7 kbps speech channel) One-eighth rate traffic channel (used for low-rate signalling channels, data channels, common channels) Associated Signalling Channels SACCH (slow associated control channel) Used for call-associated signalling, particularly measurement data needed for handover decisions A TCH is always allocated with an associated SACCH The TCH plus SACCH combination is designated TACH FACCH (fast associated control channel). This indicates call establishment progress, authenticates subscribers, and commands handovers, etc Makes use of a TCH A "stealing flag" on the TCH indicates whether it is being used for signalling, or for call transmission 05-16

9 Associated Signalling Channels cont d. SDCCH (stand alone dedicated control channel). This uses a TCH/8 channel, and is used solely for passing signalling information (e.g. location updating), and not for calls Common Signalling Channels Downlink channels (base station to mobile): FCCH (frequency correction channel) is used to identify a beacon frequency SCH (synchronisation channel) follows each FCCH to obtain synchronisation BCCH (broadcast control channel) is broadcast regularly and received by each mobile station while it is in the idle mode. It gives information about the cell, such as which network the cell belongs to. PAGCH (paging and access grant channel) is used to page a called mobile, and to allocate a channel during call set-up. There may be a full rate PAGCH/F or a one-third rate PAGCH/T. cont d next page 05-18

10 Common Signalling Channels cont d... Downlink channels (base station to mobile) cont d... CBCH (cell broadcast channel) can be used to transmit one 80 octet message every 2 seconds. It uses half a TCH/8 channel. How cell selection works: The MS finds the FCCH burst, then looks for an SCH burst on the same frequency to obtain frame synchronisation. The MS then receives BCCH on several time slots and selects a proper cell Common Signalling Channels cont d... Uplink channels (mobile station to base station): There is only one common access channel on the uplink RACH (random-access channel). The MS uses this channel to access the network. These may be provided as a full rate RACH/F or a half rate RACH/H

11 Multiple Access Scheme Slot length is called a burst period, or BP, and is of length 15/26 ms = ms. frequency Forward link and reverse link relations: Frequencies separated by 45 Mz for 900 MHz band 75 MHz for 1800 MHz band Reverse link time slot follows forward link time slot by 3BP 200 khz channel ms slot length (BP) time Multiple Access Scheme cont d... Traffic channels and signalling channels need to be efficiently multiplexed into this slot structure (non-trivial!) A full-rate traffic channel TCH/F consists of one slot every 8 BP => frame length is 8 BP = ms ms slot length TN0 TN1 TN2 TN3 TN4 TN5 TN6 TN7 TN0 TN1 TN2 TN3 TN4 TN5 TN6 TN ms frame length Slots within a frame are numbered TN0, TN1,,TN

12 Multiple Access Scheme cont d... Traffic channel frames are transmitted in groups of 26, known as a "26-multiframe", of length 120 ms (= ms) 26-multiframe, 120 ms TN0 TN1 TN2 TN3 TN4 TN5 TN6 TN7 Frame, 4.6 ms BP, or slot, ms Multiple Access Scheme cont d... The 26-multiframe structure allows the efficient mutiplexing of the associated signalling channels. The TCH/F is always allocated with its associated SACCH as follows: A "26-multiframe" of 26 8 BP is transmitted A single TCH/F uses one BP in 24 of the 26 frames of the 26-multiframe The associated SACCH uses one BP per 26-multiframe One slot in the multiframe is left idle (assists handover measurements) Therefore, a single TCH/F plus SACCH combination uses one BP per frame (26 BP total per 26-multiframe) Note: SACCH associated with a TCH/F consists of 1 slot every 120 ms

13 Common Signalling Channels Common channels are based on a cycle of 51 frames, i.e. a "51-multiframe, of length 235 ms Why 51? Deliberately different from the 26 used for traffic channels To allow MS to listen to SCH and FCCH of surrounding BSs, as needed for handoff Common Signalling Channels Downlink: The FCCH and SCH between them use 10 slots per cycle of 51 frames. FCCH uses every 10th slot in a cycle (a slot in frames 0, 10, 20, 30 40) SCH uses a slot one frame after each FCCH slot (a slot in frames 1,11,21,31,41) The BCCH and PAGCH/F together use 40 slots per 51 multiframe; BCCH in frames 2,3,4,5 and PAGCH/F in frames 6-9,12-19, 22-29, 32-39, multiframe, one slot per frame shown 05-26

14 Common Signalling Channels Uplink: A RACH/F uses one slot every frame Uplink: RACH/F: one slot per frame 51-multiframe, one slot per frame shown Fractional Rate Channels Traffic Channels: The half-rate traffic channel TCH/H and one-eighth rate traffic channel TCH/8 use similar ideas to TCH/F, but are slightly more complex. They are both always allocated with an associated SACCH. Forward Link Common Signalling Channels: The BCCH and one-third rate paging channel PAGCH/T together use 16 slots per 51 multiframe: BCCH in frames 2,3,4,5 and PAGCH/F in frames 6-9,12-19 Reverse Link Common Signalling Channels: A half-rate random access channel RACH/H uses 27 slots in the cycle a slot in frames 4,5,14-36, 45,46 (allows grouping with 4 TACH/8, i.e. 4 (TCH/8 plus its SACCH)) 05-28

15 Channel Organisation in a Cell Several of the signalling/control channels can be grouped together so that they make use of one slot per frame. For example, one slot per frame could be used for: 1 (TCH/F plus associated SACCH) 2 (TCH/H plus associated SACCH) 8 (TCH/8 plus associated SACCH) (1 SCH + 1 FCCH + 1 BCCH + 1 PAGCH/F) on the downlink + 1 RACH/F on the uplink (1 BCCH + 1 PAGCH/F) on the downlink + 1 RACH/F on the uplink 1 BCCH + 1 PAGCH/T on the downlink + 1 RACH/H on the uplink + 4 (TCH/8 plus associated SACCH) using both uplink and downlink. etc Traffic Channel Combinations Signalling Channel Combinations Both Traffic and Signalling Example Channel Organisation in a Cell For example, a combination of 1 SCH + 1 FCCH + 1 BCCH + 1 PAGCH/F on the downlink uses (per 51 multiframe): Downlink: FCCH: slot in frames 0, 10, 20, 30, 40 SCH: slot in frames 1, 11, 21, BCCH: slot in frames 2, 3, 4, 5 PAGCH/F: slot in frames 6-9, 12-19, 22-29, 32-39, Uplink: RACH/F: one slot per frame 05-30

16 Example: Small Capacity Cell One TRX, consisting of: TN 0: FCCH, SCH, BCCH, PAGCH/T, RACH/H, 4 (TCH/8 plus associated SACCH) TN 1 to 7: 1 (TCH/F plus associated SACCH) frequency TCH/F + SACCH Signalling Channel time Example: Medium Capacity Cell Four TRXs, consisting of: One TN 0 group: FCCH, SCH, BCCH, PAGCH/F, RACH/F Two sets of 8 (TCH/8 plus associated SACCH) 29 (TCH/F plus associated SACCH) frequency Signalling Channels TCH/8 + SACCH 29 Remaining slots TCH/F + SACCH time 05-32

17 Example: Large Capacity Cell Twelve TRXs, consisting of: One TN 0 group: FCCH, SCH, BCCH, PAGCH/F, RACH/F One TN 2 group: BCCH, PAGCH/F, RACH/F One TN 4 group: BCCH, PAGCH/F, RACH/F One TN 6 group: BCCH, PAGCH/F, RACH/F Five sets of 8 (TCH/8 plus associated SACCH) 87 (TCH/F plus associated SACCH) frequency Signalling Channels TCH/8 + SACCH TCH/8 + SACCH 87 Remaining slots TCH/F + SACCH time Summary of GSM Frames Frame = 8 BP = ms 26 multiframe = 26 8 BP = 120 ms multiplexes traffic channels plus their associated control channels 51 multiframe = 51 8 BP = ms multiplexes the common control channels Superframe = BP =6.12 s smallest cycle for which channel organisation is repeated Hyperframe = 2048 superframes numbering period 05-34

18 References GSM Architecture Mouly & Pautet, Chapter 2 Redl et al, Section 3.4 Lee, pp Rappaport, Section GSM Channel Structure Mouly & Pautet, Section Redl et al, Section Lee, pp Rappaport, Sections